Journal articles on the topic 'Centre vortices'
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1
Biddle, James, Waseem Kamleh, and Derek Leinweber. "Visualisations of Centre Vortices." EPJ Web of Conferences 245 (2020): 06010. http://dx.doi.org/10.1051/epjconf/202024506010.
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The centre vortex structure of the vacuum is visualised through the use of novel 3D visualisation techniques. These visualisations allow for a hands-on examination of the centre-vortex matter present in the QCD vacuum, and highlights some of the key features of the centre-vortex model. The connection between topological charge and singular points is also explored. This work highlights the useful role visualisations play in the exploration of the QCD vacuum.
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Stephenson, P. "Centre vortices and their friends." Nuclear Physics B - Proceedings Supplements 83-84, no. 1-3 (March 2000): 544–46. http://dx.doi.org/10.1016/s0920-5632(00)00302-9.
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Stephenson, P. W. "Centre vortices and their friends." Nuclear Physics B - Proceedings Supplements 83-84 (April 2000): 544–46. http://dx.doi.org/10.1016/s0920-5632(00)91733-x.
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Wang, Jinjun, Chong Pan, Kwing-So Choi, Lei Gao, and Qi-Xiang Lian. "Formation, growth and instability of vortex pairs in an axisymmetric stagnation flow." Journal of Fluid Mechanics 725 (May 23, 2013): 681–708. http://dx.doi.org/10.1017/jfm.2013.205.
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AbstractThe formation, growth and instability of a pair of counter-rotating vortices over a circular plate in the downstream of a thin fishing line were studied using particle image velocimetry and flow visualization. Initially, the vortex pair in an axisymmetric stagnation flow was small, but it grew steadily by accumulating the shear-layer vorticity of the wake before going through vortical instability. Two types of vortical development were observed in the present experiment. Type I was a common type of vortical development in an axisymmetric stagnation flow over a circular plate. Here, the circulation of the vortex pair increased linearly with time reflecting a constant flux of vorticity impinging on the plate wall. After the growth, the counter-rotating pair of vortices went through an antisymmetric deformation in the wall-normal direction while the vortex deformation was symmetric in the wall-parallel direction. This was remarkably similar to the short-wavelength elliptic instability of counter-rotating vortices in an open system. On the other hand, type II development of a vortex pair was a rare case, where the vortices grew for much longer duration than in type I cases. This initiated a breakdown of vortices before the residual vorticity moved away from the centre of the plate. It is considered that the disturbance due to vortical instability could be partially responsible for the unexpectedly high heat transfer rate in the stagnation region of bluff bodies that has been reported in the last half-century.
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Leinweber, Derek, James Biddle, Waseem Kamleh, and Adam Virgili. "Dynamical fermions, centre vortices, and emergent phenomena." EPJ Web of Conferences 274 (2022): 01002. http://dx.doi.org/10.1051/epjconf/202227401002.
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The non-trivial ground-state vacuum fields of QCD form the foundation of matter. Here we examine the centre vortices identified within the ground-state fields of lattice QCD. We aim to understand the manner in which dynamical fermions in the QCD vacuum alter the centre-vortex structure. Using modern visualisation techniques, the centre-vortex structure of pure-gauge and dynamical-fermion fields is quantified and compared. We then explore the impact this modified structure has on measures of confinement and dynamical mass generation. The string tension of the static quark potential, positivityviolation in the gluon propagator, and dynamical mass generation in the overlap quark propagator are of particular interest. The impact of dynamical fermions is significant and provides new insights into the role of centre vortices in underpinning both confinement and dynamical chiral symmetry breaking in QCD.
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Leinweber, Derek B., Patrick O. Bowman, Urs M. Heller, Daniel-Jens Kusterer, Kurt Langfeld, and Anthony G. Williams. "Role of centre vortices in dynamical mass generation." Nuclear Physics B - Proceedings Supplements 161 (November 2006): 130–35. http://dx.doi.org/10.1016/j.nuclphysbps.2006.08.065.
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VAN DE KONIJNENBERG, J. A., and G. J. F. VAN HEIJST. "Free-surface effects on spin-up in a rectangular tank." Journal of Fluid Mechanics 334 (March 10, 1997): 189–210. http://dx.doi.org/10.1017/s0022112096004296.
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The dependence of spin-up in a rectangular tank on deformation of the free surface is investigated experimentally. The results agree with earlier experimental and numerical data about the motion of vortices over topography. However, the presence of sidewalls appears to interact with the vortex drift induced by the surface topography. This combined effect provides a qualitative explanation for the observed behaviour of individual vortices. In particular, in the presence of free-surface deformation, cyclonic vortices in an elongated rectangle tend to drift away from the centre of the tank, so that their merging in the centre is discouraged.
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DAVYDOVA, T. A., and V. M. LASHKIN. "Drift-wave trapping by drift vortices." Journal of Plasma Physics 58, no. 1 (July 1997): 11–18. http://dx.doi.org/10.1017/s002237789700562x.
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The possibility for a drift dipole vortex to trap free drift waves is demonstrated. Drift perturbations can be trapped near the centre of the vortex or at its sides. The localization domain and eigenfrequencies of trapped modes are obtained.
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Griffiths, R. W., and E. J. Hopfinger. "Experiments with baroclinic vortex pairs in a rotating fluid." Journal of Fluid Mechanics 173 (December 1986): 501–18. http://dx.doi.org/10.1017/s0022112086001246.
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When vortices are generated in one layer of a rotating, two-layer density stratification, the velocity field of each vortex is strongly baroclinic within a distance of order one Rossby radius from its centre. In this system there are two classes of vortex pairs: those pairs (consisting of vortices of opposite signs) for which the vortices are in the same layer, and those for which the vortices are in opposite layers. We pay particular attention to a laboratory demonstration of the properties of the latter class. These vortex pairs have the ability to transport density (or heat) in the horizontal, and provide a means for describing the release of potential energy by baroclinic instability. We also observe that interactions of real vortices and real vortex pairs differ from those computed for point vortices.
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Nycander, J. "The difference between monopole vortices in planetary flows and laboratory experiments." Journal of Fluid Mechanics 254 (September 1993): 561–77. http://dx.doi.org/10.1017/s0022112093002253.
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This work is an attempt to explain observations of vortices in experiments with shallow water in rotating paraboloidal vessels. The most long-lived vortices are invariably anticyclones, while cyclones quickly disperse, and they are larger than the Rossby radius. These experiments are designed to simulate geophysical flows, where large, long-lived, anticyclonic vortices are common.The general condition for vortices to be steady is that they propagate faster than linear Rossby waves, so that the vortex energy is not dispersed by coupling to linear waves. The propagation velocity is determined by a general integral relation that gives the velocity of the centre of mass. In geophysical flows, to lowest order in the Rossby number, the difference between the centre-of-mass velocity and the maximum phase velocity of the Rossby waves is proportional to the relative perturbation of the fluid depth. Since for anticyclones the difference is positive they may be steady, whereas cyclones cannot be.In the laboratory experiments this velocity difference is absent because of the latitudinal dependence of the effective gravity caused by the centrifugal force. However, to the next order in the Rossby number, there is another nonlinear contribution, so that anticyclones (but not cyclones) still propagate faster than the linear Rossby waves, and may thus be steady. The velocity difference is smaller than for geophysical flows, and vanishes in the limit of small Rossby number. The existence conditions also show that we can expect the experimental vortices to be smaller (as measured by the Rossby radius) than the planetary vortices. The theory does not apply to vortices that are much smaller than the Rossby radius.
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Shahmardi, Armin, Sagar Zade, Mehdi N. Ardekani, Rob J. Poole, Fredrik Lundell, Marco E. Rosti, and Luca Brandt. "Turbulent duct flow with polymers." Journal of Fluid Mechanics 859 (November 28, 2018): 1057–83. http://dx.doi.org/10.1017/jfm.2018.858.
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We have performed direct numerical simulation of the turbulent flow of a polymer solution in a square duct, with the FENE-P model used to simulate the presence of polymers. First, a simulation at a fixed moderate Reynolds number is performed and its results compared with those of a Newtonian fluid to understand the mechanism of drag reduction and how the secondary motion, typical of the turbulent flow in non-axisymmetric ducts, is affected by polymer additives. Our study shows that the Prandtl’s secondary flow is modified by the polymers: the circulation of the streamwise main vortices increases and the location of the maximum vorticity moves towards the centre of the duct. In-plane fluctuations are reduced while the streamwise ones are enhanced in the centre of the duct and dumped in the corners due to a substantial modification of the quasi-streamwise vortices and the associated near-wall low- and high-speed streaks; these grow in size and depart from the walls, their streamwise coherence increasing. Finally, we investigated the effect of the parameters defining the viscoelastic behaviour of the flow and found that the Weissenberg number strongly influences the flow, with the cross-stream vortical structures growing in size and the in-plane velocity fluctuations reducing for increasing flow elasticity.
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Reinaud, Jean N. "Three-dimensional quasi-geostrophic vortex equilibria with -fold symmetry." Journal of Fluid Mechanics 863 (January 22, 2019): 32–59. http://dx.doi.org/10.1017/jfm.2018.989.
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We investigate arrays of $m$ three-dimensional, unit-Burger-number, quasi-geostrophic vortices in mutual equilibrium whose centroids lie on a horizontal circular ring; or$m+1$ vortices where the additional vortex lies on the vertical ‘central’ axis passing through the centre of the array. We first analyse the linear stability of circular point vortex arrays. Three distinct categories of vortex arrays are considered. In the first category, the $m$ identical point vortices are equally spaced on a circular ring and no vortex is located on the vertical central axis. In the other two categories, a ‘central’ vortex is added. The latter two categories differ by the sign of the central vortex. We next turn our attention to finite-volume vortices for the same three categories. The vortices consist of finite volumes of uniform potential vorticity, and the equilibrium vortex arrays have an (imposed) $m$-fold symmetry. For simplicity, all vortices have the same volume and the same potential vorticity, in absolute value. For such finite-volume vortex arrays, we determine families of equilibria which are spanned by the ratio of a distance separating the vortices and the array centre to the vortices’ mean radius. We determine numerically the shape of the equilibria for $m=2$ up to $m=7$, for each three categories, and we address their linear stability. For the $m$-vortex circular arrays, all configurations with $m\geqslant 6$ are unstable. Point vortex arrays are linearly stable for $m<6$. Finite-volume vortices may, however, be sensitive to instabilities deforming the vortices for $m<6$ if the ratio of the distance separating the vortices to their mean radius is smaller than a threshold depending on $m$. Adding a vortex on the central axis modifies the overall stability properties of the vortex arrays. For $m=2$, a central vortex tends to destabilise the vortex array unless the central vortex has opposite sign and is intense. For $m>2$, the unstable regime can be obtained if the strength of the central vortex is larger in magnitude than a threshold depending on the number of vortices. This is true whether the central vortex has the same sign as or the opposite sign to the peripheral vortices. A moderate-strength like-signed central vortex tends, however, to stabilise the vortex array when located near the plane containing the array. On the contrary, most of the vortex arrays with an opposite-signed central vortex are unstable.
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LE DIZÈS, STÉPHANE, and DAVID FABRE. "Large-Reynolds-number asymptotic analysis of viscous centre modes in vortices." Journal of Fluid Mechanics 585 (August 7, 2007): 153–80. http://dx.doi.org/10.1017/s002211200700660x.
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This paper presents a large-Reynolds-number asymptotic analysis of viscous centre modes on an arbitrary axisymmetrical vortex with an axial jet. For any azimuthal wavenumber m and axial wavenumber k, the frequency of these modes is given at leading order by ω0 = mΩ0 + kW0 where Ω0 and W0 are the angular and axial velocities of the vortex at its centre. These modes possess a multi-layer structure localized in an O(Re−1/6) neighbourhood of the vortex. By a multiple-scale matching analysis, we demonstrate the existence of three different families of viscous centre modes whose frequency expands as ω(n) ∼ ω0 + Re−1/3ω1 + Re−1/2ω(n)2. One of these families is shown to have unstable eigenmodes when H0 = 2Ω0k(2kΩ0 − mW2) < 0 where W2 is the second radial derivative of the axial flow in the centre. The growth rate of these modes is given at leading order by σ ∼ (3/2)(H0/4)1/3Re−1/3. Our results prove that any vortex with a jet (or jet with swirl) such that Ω0W2 ≠ 0 is unstable if the Reynolds number is sufficiently large. The spatial structure of the viscous centre modes is obtained and simple approximations which capture the main feature of the eigenmodes are also provided.The theoretical predictions are compared with numerical results for the q-vortex model (or Batchelor vortex) for Re ≥ 105. For all modes, a good agreement is demonstrated for both the frequency and the spatial structure.
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Vernet, Julie A., Ramis Örlü, and P. Henrik Alfredsson. "Flow separation control behind a cylindrical bump using dielectric-barrier-discharge vortex generator plasma actuators." Journal of Fluid Mechanics 835 (November 28, 2017): 852–79. http://dx.doi.org/10.1017/jfm.2017.773.
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Dielectric-barrier-discharge plasma actuators are arranged to produce counter-rotating streamwise vortices to control flow separation on a cylindrical bump on a flat plate that is approached by a turbulent boundary layer. The control was tested for different free-stream velocities and actuation driving voltages. The recirculation area downstream of the bump was reduced by the actuation for velocities up to $15~\text{m}~\text{s}^{-1}$ at the highest voltage achievable of the present set-up. However, the flow shows a bi-modality, the nominal two-dimensional wake flow is shown to consist of large-scale streamwise vortices, which are energised by the actuation until a phenomenon of lock-on of these vortices occurs at sufficiently high driving voltages. The wavelength of the actuation is half that of the large-scale vortices. The lock-on shifts sometimes, i.e. the large streamwise vortices centre switch spanwise location, explaining the bi-modality in the flow. The details of the bi-modality are further investigated by conditional averaging and proper orthogonal decomposition.
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Carnevale, G. F., R. C. Kloosterziel, and G. J. F. Van Heijst. "Propagation of barotropic vortices over topography in a rotating tank." Journal of Fluid Mechanics 233 (December 1991): 119–39. http://dx.doi.org/10.1017/s0022112091000411.
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A small-scale cyclonic vortex in a relatively broad valley tends to climb up and out of the valley in a cyclonic spiral about the centre, and when over a relatively broad hill it tends to climb toward the top in an anticyclonic spiral around the peak. This phenomenon is examined here through two-dimensional numerical simulations and rotating-tank experiments. The basic mechanism involved is shown to be the same as that which accounts for the northwest propagation of cyclones on a β-plane. This inviscid nonlinear effect is also shown to be responsible for the observed translationary motion of barotropic vortices in a free-surface rotating tank. The behaviour of isolated vortices is contrasted with that of vortices with non-vanishing circulation.
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Reinaud, Jean N., and David G. Dritschel. "The stability and nonlinear evolution of quasi-geostrophic toroidal vortices." Journal of Fluid Mechanics 863 (January 22, 2019): 60–78. http://dx.doi.org/10.1017/jfm.2018.1013.
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We investigate the linear stability and nonlinear evolution of a three-dimensional toroidal vortex of uniform potential vorticity under the quasi-geostrophic approximation. The torus can undergo a primary instability leading to the formation of a circular array of vortices, whose radius is approximately the same as the major radius of the torus. This occurs for azimuthal instability mode numbers $m\geqslant 3$, on sufficiently thin tori. The number of vortices corresponds to the azimuthal mode number of the most unstable mode growing on the torus. This value of $m$ depends on the ratio of the torus’ major radius to its minor radius, with thin tori favouring high mode $m$ values. The resulting array is stable when $m=4$ and $m=5$ and unstable when $m=3$ and $m\geqslant 6$. When $m=3$ the array has barely formed before it collapses towards its centre with the ejection of filamentary debris. When $m=6$ the vortices exhibit oscillatory staggering, and when $m\geqslant 7$ they exhibit irregular staggering followed by substantial vortex migration, e.g. of one vortex to the centre when $m=7$. We also investigate the effect of an additional vortex located at the centre of the torus. This vortex alters the stability properties of the torus as well as the stability properties of the circular vortex array formed from the primary toroidal instability. We show that a like-signed central vortex may stabilise a circular $m$-vortex array with $m\geqslant 6$.
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He, Xinyu. "A probabilistic method for Navier-Stokes vortices." Journal of Applied Probability 38, no. 4 (December 2001): 1059–66. http://dx.doi.org/10.1239/jap/1011994192.
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Consider a Navier-Stokes incompressible turbulent fluid in R2. Let x(t) denote the position coordinate of a moving vortex with initial circulation Γ0 > 0 in the fluid, subject to a force F. Define x(t) as a stochastic process with continuous sample paths described by a stochastic differential equation. Assuming a suitable notion of weak rotationality, it is shown that the stochastic equation is equivalent to a linear partial differential equation for the complex function ψ, i∂ψ/∂t = [-Γ0Δ + F] ψ, where |ψ|2 = ρ(x,t), ρ being the probability density function of finding the vortex centre in position x at time t.
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He, Xinyu. "A probabilistic method for Navier-Stokes vortices." Journal of Applied Probability 38, no. 04 (December 2001): 1059–66. http://dx.doi.org/10.1017/s0021900200019239.
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Consider a Navier-Stokes incompressible turbulent fluid in R 2. Let x(t) denote the position coordinate of a moving vortex with initial circulation Γ0 > 0 in the fluid, subject to a force F. Define x(t) as a stochastic process with continuous sample paths described by a stochastic differential equation. Assuming a suitable notion of weak rotationality, it is shown that the stochastic equation is equivalent to a linear partial differential equation for the complex function ψ, i∂ψ/∂t = [-Γ0Δ + F] ψ, where |ψ|2 = ρ(x,t), ρ being the probability density function of finding the vortex centre in position x at time t.
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Biddle, James, Waseem Kamleh, and Derek Leinweber. "Emergent Structure in QCD." EPJ Web of Conferences 245 (2020): 06009. http://dx.doi.org/10.1051/epjconf/202024506009.
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The structure of the S U(3) gauge-field vacuum is explored through visualisations of centre vortices and topological charge density. Stereoscopic visualisations highlight interesting features of the vortex vacuum, especially the frequency with which singular points appear and the important connection between branching points and topological charge. This work demonstrates how visualisations of the QCD ground-state fields can reveal new perspectives of centre-vortex structure.
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HU, J. C., and Y. ZHOU. "Flow structure behind two staggered circular cylinders. Part 1. Downstream evolution and classification." Journal of Fluid Mechanics 607 (June 30, 2008): 51–80. http://dx.doi.org/10.1017/s0022112008001808.
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Flow structures, Strouhal numbers and their downstream evolutions in the wake of two-staggered circular cylinders are investigated at Re=7000 using hot-wire, flow-visualization and particle-image velocimetry techniques. The cylinder centre-to-centre pitch, P, ranges from 1.2d to 4.0d (d is the cylinder diameter) and the angle (α) between the incident flow and the line through the cylinder centres is 0° ~ 90°. Four distinct flow structures are identified at x/d ≥ 10 (x is the downstream distance from the mid-point between the cylinders), i.e. two single-street modes (S-I and S-II) and two twin-street modes (T-I and T-II), based on Strouhal numbers, flow topology and their downstream evolution. Mode S-I is further divided into two different types, i.e. S-Ia and S-Ib, in view of their distinct vortex strengths. Mode S-Ia occurs at P/d ≤ 1.2. The pair of cylinders behaves like one single body, and shear layers separated from the free-stream sides of the cylinders roll up, forming one street of alternately arranged vortices. The street is comparable to that behind an isolated cylinder in terms of the topology and strength of vortices. Mode S-Ib occurs at α ≤ 10° and P/d > 1.5. Shear layers separated from the upstream cylinder reattach on or roll up to form vortices before reaching the downstream cylinder, resulting in postponed flow separation from the downstream cylinder. A single vortex street thus formed is characterized by significantly weakened vortices, compared with Mode S-Ia. Mode S-II is identified at P/d=1.2~2.5 and α>20° or 1.5≤P/d≤4.0 and 10° < α≤20°, where both cylinders generate vortices, with vortex shedding from the upstream cylinder at a much higher frequency than from the downstream, producing two streets of different widths and vortex strengths at x/d≤5.0. The two streets interact vigorously, resulting in a single street of the lower-frequency vortices at x/d≥10. The vortices generated by the downstream cylinder are significantly stronger than those, originating from the upstream cylinder, in the other row. Mode T-I occurs at P/d≥2.5 and α=20°~88°; the two cylinders produce two streets of different vortex strengths and frequencies, both persisting beyond x/d=10. At P/d≥2.5 and α≥88°, the two cylinders generate two coupled streets, mostly anti-phased, of the same vortex strength and frequency (St≈0.21), which is referred to as Mode T-II. The connection of the four modes with their distinct initial conditions, i.e. interactions between shear layers around the two cylinders, is discussed.
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Huang, Yuqi, James Venning, Mark C. Thompson, and John Sheridan. "Vortex separation and interaction in the wake of inclined trapezoidal plates." Journal of Fluid Mechanics 771 (April 20, 2015): 341–69. http://dx.doi.org/10.1017/jfm.2015.160.
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Full three-dimensional numerical simulations are employed to investigate the flows over inclined trapezoidal low-aspect-ratio plates at low Reynolds numbers, aiming to understand the unsteadiness induced by the interaction between the trailing vortical wake structures originating from the swept edges, and those from the leading and trailing edges. The flows past eighteen different plate geometries in three broad sets are simulated to study the influence of aspect ratio, taper angle and angle of attack on the wake vortices and the force coefficients. Both taper ratio and angle of attack of plates with the same area are found to have a broadly predictable influence on the wake stability and asymptotic forces. Smaller taper ratios result in lower maximum lift, while an increase in the angle of attack results in a reduction in the differences in maximum lift. Two distinct modes of periodic unsteady flow with significant differences in frequency are observed. The corresponding vortex-shedding mechanisms are analysed with the aid of $Q$-criterion isosurfaces and streamlines. A low wake frequency is observed at small taper angles when there is relative independence between the von Kármán vortices originating from the leading and trailing edges, and weak swept-edge vortices. The dominant Strouhal number in this state is approximately 0.09. When the taper angle or angle of attack increases, the flows over the swept edges form stronger trailing vortex structures which interact strongly with the leading-edge vortices, combining to produce a regular stream of vortex loops shed into the wake. In this regime, the dominant Strouhal number increases to approximately 0.14–0.18. Higher Reynolds numbers and/or angles of attack result in a loss of centre plane reflection symmetry in the wake. The aerodynamic forces have been quantified as a function of the problem parameters and plate geometry.
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Zheng, Z. C., and W. Li. "Dependence of radiated sound frequency on vortex core dynamics in multiple vortex interactions." Aeronautical Journal 113, no. 1142 (April 2009): 233–42. http://dx.doi.org/10.1017/s0001924000002906.
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Abstract With both theoretical analysis and measurement data, it has been identified previously that there exists a robust sound emission from a pair of counter-rotating aircraft wake vortices at the frequency of unsteady vortex core rotation. In a vortex system with multiple vortices, the sound emission frequency can be subjected to change because of interactions among the vortices. The behaviour of the influence, indicated by the ratio between the core size and the distance of the vortices and the underlining vortex core dynamic mechanisms, is investigated in this study. A vortex particle method is used to simulate the vortex core dynamics in two-dimensional, inviscid and incompressible flow. The flow field, in the form of vorticity, is employed as the source in the far-field acoustic calculation using a vortex sound formula. Cases of co-rotating vortices and a multiple-vortex system composed of two counter-rotating vortex pairs are studied for applications to aircraft wake vortex sound. The study shows, without vortex merging, individual frequencies can be clearly identified that are due each to core rotation (self induction) and co-rotating motion of a vortex centre around the other (mutual induction). The ratio of the core size and the distance between vortices does not seem to significantly influence the frequency of vortex core rotation. With vortex merging, a single frequency due to the merged vortex core is generated.
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Carnevale, G. F., and R. C. Kloosterziel. "Emergence and evolution of triangular vortices." Journal of Fluid Mechanics 259 (January 25, 1994): 305–31. http://dx.doi.org/10.1017/s0022112094000157.
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Laboratory observations and numerical simulations reveal that, in addition to monopoles, dipoles and tripoles, yet another stable coherent vortex may emerge from unstable isolated circular vortices. This new vortex is the finite-amplitude result of the growth of an azimuthal wavenumber-3 perturbation. It consists of a triangular core of single-signed vorticity surrounded by three semicircular satellites of oppositely signed vorticity. The stability of this triangular vortex is analysed through a series of high-resolution numerical simulations and by an investigation of point-vortex models. This new compound vortex rotates about its centre and is stable to small perturbations. If perturbed strongly enough, it undergoes an instability in which two of the outer satellites merge, resulting in the formation of an axisymmetric tripole, which subsequently breaks down into either a pair of dipoles or a dipole plus a monopole. The growth of higher-azimuthal-wavenumber perturbations leads to the formation of more intricate compound vortices with cores in the shape of squares, pentagons, etc. However, numerical simulations show that these vortices are unstable, which agrees with results from point-vortex models.
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Bandyopadhyay, Promode R. "Aspects of the equilibrium puff in transitional pipe flow." Journal of Fluid Mechanics 163 (February 1986): 439–58. http://dx.doi.org/10.1017/s0022112086002379.
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Flow-visualization studies in transitional pipe flow are used to reveal the mechanism responsible for the self-sustenance of a turbulent equilibrium puff. The upstream laminar fluid continuously enters the relatively-slower-moving turbulent puff around the pipe centre. The passage of this high-speed laminar plug flow past the slower fluid that resides near the wall at the upstream interface leads to the shedding of a train of three-dimensional wake-like vortices near the wall. A helical motion near the upstream interface is associated with the vortex-shedding process. The remainder of the puff is a cone of turbulence filled with these wake-like vortices that are decaying slowly; the prominent feature of the decay region is the longitudinal vortices that are apparently undergoing stretching. No toroidal vortex has been observed in the instantaneous flow field at the upstream interface of an individual puff. On the other hand, the wake-like vortices reported here have not been observed before because their three-dimensional and random nature does not allow detection by an ensemble-averaging that is not phase-referenced appropriately.
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Ohring, Samuel, and Hans J. Lugt. "Interaction of a viscous vortex pair with a free surface." Journal of Fluid Mechanics 227 (June 1991): 47–70. http://dx.doi.org/10.1017/s0022112091000034.
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A vortex pair in a viscous, incompressible fluid rises vertically toward a deformable free surface. The mathematical, description of this flow situation is a time-dependent nonlinear free-surface problem that has been solved numerically for a two-dimensional laminar flow with the aid of the Navier-Stokes equations by using boundary-fitted coordinates. For a number of selected flow parameters, results are presented on the decay of the primary vortices and their paths, the generation of surface vorticity and secondary vortices, the development and final stage of the disturbed free surface, and the influence of surface tension. High and low Froude numbers represent the two extremes of free-surface yielding and stiffness, respectively. For an intermediate Froude number, a special rebounding due to the presence of secondary vortices has been observed: the path of the primary vortex centre portrays a complete loop.
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Newton, Paul K., and Takashi Sakajo. "The N -vortex problem on a rotating sphere. III. Ring configurations coupled to a background field." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 463, no. 2080 (January 9, 2007): 961–77. http://dx.doi.org/10.1098/rspa.2006.1802.
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We study the evolution of N -point vortices in ring formation embedded in a background flowfield that initially corresponds to solid-body rotation on a sphere. The evolution of the point vortices is tracked numerically as an embedded dynamical system along with the M contours which separate strips of constant vorticity. The full system is a discretization of the Euler equations for incompressible flow on a rotating spherical shell, hence a ‘barotropic’ model of the one-layer atmosphere. We describe how the coupling creates a mechanism by which energy is exchanged between the ring and the background, which ultimately serves to break up the structure. When the centre-of-vorticity vector associated with the ring is initially misaligned with the axis of rotation of the background field, it sets up the propagation of Rossby waves around the sphere which move retrograde to the solid-body rotation. These waves pass energy to the ring (in the case when the solid-body field and the ring initially co-rotate) or extract energy from the ring (when the solid-body field and the ring initially counter-rotate), hence the Hamiltonian and the centre-of-vorticity vector associated with the N -point vortices are no longer conserved as they are for the one-way coupled model described by Newton & Shokraneh. In the first case, energy is transferred to the ring, the length of the centre-of-vorticity vector increases, while its tip spirals in a clockwise manner towards the North Pole. The ring stays relatively intact for short times, but ultimately breaks-up on a longer time-scale. In the latter case, energy is extracted from the ring, the length of the centre-of-vorticity vector decreases while its tip spirals towards the North Pole and the ring loses its coherence more quickly than in the co-rotating case. The special case where the ring is initially oriented so that its centre-of-vorticity vector is perpendicular to the axis of rotation is also examined as it shows how the coupling to the background field breaks this symmetry. In this case, both the length of the centre-of-vorticity vector and the Hamiltonian energy of the ring achieve a local maximum at roughly the same time.
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CHEN, A. L., J. D. JACOB, and Ö. SAVAŞ. "Dynamics of corotating vortex pairs in the wakes of flapped airfoils." Journal of Fluid Mechanics 382 (March 10, 1999): 155–93. http://dx.doi.org/10.1017/s0022112098003814.
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The behaviour of a pair of corotating vortices in the wake of a flapped airfoil is experimentally studied in a water towing tank. Reynolds numbers based on total circulation of the vortices range from 1.0×104 to 6.4×104. Planar velocity vector fields and their gradients are derived from PIV images using an adaptive Lagrangian parcel tracking algorithm. Isovorticity surfaces are extracted from time series of planar vorticity data. The behaviour of the vortices is tracked by using various moments of both the probability density distribution and the spatial distribution of their streamwise vorticity. All vortices show a Lamb–Oseen circulation distribution when they are clearly identifiable. Further, vortices from the apless wing exhibit Lamb–Oseen velocity and vorticity distributions with slow growth. All corotating vortex pairs are observed to merge at about 0.8 orbit periods. First-order statistics of the flow field remain invariant during the merger. The higher-order moments of the vorticity distribution show strong time dependence, which implies three-dimensionality of the flow resulting from vortex stretching. The strengths of the individual vortices before merger are constant, and the total circulation before and after merger remains constant within the range of observations. The trajectory of the centre of vorticity remains unaffected by the merger process. The merger is preceded by a splitting of the weaker vortex into filaments which, depending on the relative strengths of the vortices, can occur in the radial direction, the axial direction, or a combination of the two. Mechanisms contributing to the merger dynamics are discussed.
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Chian, Abraham C.-L., Suzana S. A. Silva, Erico L. Rempel, Milan Gošić, Luis R. Bellot Rubio, Kanya Kusano, Rodrigo A. Miranda, and Iker S. Requerey. "Supergranular turbulence in the quiet Sun: Lagrangian coherent structures." Monthly Notices of the Royal Astronomical Society 488, no. 3 (July 11, 2019): 3076–88. http://dx.doi.org/10.1093/mnras/stz1909.
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ABSTRACT The quiet Sun exhibits a wealth of magnetic activities that are fundamental for our understanding of solar magnetism. The magnetic fields in the quiet Sun are observed to evolve coherently, interacting with each other to form prominent structures as they are advected by photospheric flows. The aim of this paper is to study supergranular turbulence by detecting Lagrangian coherent structures (LCS) based on the horizontal velocity fields derived from Hinode intensity images at disc centre of the quiet Sun on 2010 November 2. LCS act as transport barriers and are responsible for attracting/repelling the fluid elements and swirling motions in a finite time. Repelling/attracting LCS are found by computing the forward/backward finite-time Lyapunov exponent (FTLE), and vortices are found by the Lagrangian-averaged vorticity deviation method. We show that the Lagrangian centres and boundaries of supergranular cells are given by the local maximum of the forward and backward FTLE, respectively. The attracting LCS expose the location of the sinks of photospheric flows at supergranular junctions, whereas the repelling LCS interconnect the Lagrangian centres of neighbouring supergranular cells. Lagrangian transport barriers are found within a supergranular cell and from one cell to other cells, which play a key role in the dynamics of internetwork and network magnetic elements. Such barriers favour the formation of vortices in supergranular junctions. In particular, we show that the magnetic field distribution in the quiet Sun is determined by the combined action of attracting/repelling LCS and vortices.
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Zhao, Ming, and Lin Lu. "Numerical simulation of flow past two circular cylinders in cruciform arrangement." Journal of Fluid Mechanics 848 (June 13, 2018): 1013–39. http://dx.doi.org/10.1017/jfm.2018.380.
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Flow past two circular cylinders in cruciform arrangement is simulated by direct numerical simulations for Reynolds numbers ranging from 100 to 500. The study is aimed at investigating the local flow pattern near the gap between the two cylinders, the global vortex shedding flow in the wake of the cylinders and their effects on the force coefficients of the two cylinders. The three identified local flow patterns near the gap: trail vortex (TV), necklace vortex (NV) and vortex shedding in the gap (SG) agree with those found by flow visualization in experimental studies. As for the global wake flow, two modes of vortex shedding are identified: K mode with inclined wake vortices and P mode where the wake vortices are parallel to the cylinders. The K mode occurs when the gap is slightly greater than the boundary gap between the NV and SG. It also coexists with the SG gap flow pattern if the Reynolds number is very small ($Re=100$). The flow pattern affects the force coefficient. The K mode increases the mean drag coefficient and the standard deviation of the lift coefficient at the centre of the upstream cylinder because the wake vortices converge towards the centre. The mean drag coefficient and standard deviation of the lift coefficient of the downstream cylinder decreases because of the shedding effect from the upstream cylinder.
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Steponavičienė, Lina, and Artūras Jukna. "COHERENT VORTEX MOTION IN YBA2CU3O7-Z SUPERCONDUCTING THIN FILMS CONTAINING Π-SHAPED CHANNEL FOR EASY VORTEX MOTION BIASED AT I ~ IC / MAGNETINIŲ SŪKURIŲ KOHERENTINIO JUDĖJIMO YBA2CU3O7-X SUPERLAIDŽIUOSIUOSE PLONUOSIUOSE SLUOKSNIUOSE SU LAZERIU SUFORMUOTU." Mokslas - Lietuvos ateitis 3, no. 6 (January 3, 2012): 105–10. http://dx.doi.org/10.3846/mla.2011.123.
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The current vs. voltage dependences of YBa2Cu3O7-x superconducting microbridges containing laser–written (LW) -shaped channel for easy vortex motion have been investigated in a narrow range of temperatures ranging from 87.3 to 88.6 K that are still below the critical temperature Tc of the material. In the range of the before mentioned temperatures the current-self magnetic field produced, Abrikosov magnetic vortices/anti-vortices appear in the partially deoxygenated (x ~ 0.2) channel and move coherently identifying itself as Josephson–like voltage “steps” in nonlinear current–voltage dependences. Bias current creates Lorentz force FL, pushing vortices along the LW-channel from its edges towards the centre. As follows from current-voltage dependences, vortices start moving at I = Ic. This current satisfies condition FL >> Fp (where Fp – is pinning force in the film). Our results confirm that oxygen vacancies in the channel area decrease the pinning force of screw-dislocations and make the channel of uniform pinning. Then, even at I = Ic, Abrikosov magnetic vortices create a spontaneous channel in the deoxygenated channel of the superconducting microbridge and start moving coherently. Santrauka Darbe nagrinėjami silpnai tarpusavyje sąveikaujančių Abrikosovo magnetinių sūkurių judėjimo ypatumai 0,3 mm storio, 100 mm ilgio ir 50 mm pločio YBa2Cu3O7–xsuperlaidžiuosiuose dariniuose, turinčiuose iš dalies deguonimi nuskurdintą Π formos Abrikosovo magnetinių sūkurių kanalą. Sūkurių kanalas superlaidžiajame darinyje sudarytas sufokusuotu į 5 mm skersmens dėmę lazerio spinduliu. Šviesos poveikiu modifikuotame superlaidžiojo darinio kanale (x < 0,2) atsirado papildomi sūkurių prieraišos centrai (deguonies vakansijos), kurių kuriama sūkurių prieraišos jėga Fp sumažino stiprių prieraišos centrų kuriamą prieraišos jėgą ir sukūrė palankias sąlygas Abrikosovo magnetinių sūkurių judėjimui, dariniu tekant srovei, kurios tankis keliomis eilėmis mažesnis už šviesa nemodifikuoto superlaidininko dalies kritinį srovės tankį. Atliktų tyrimų rezultatai rodo, kad koherentinis Abrikosovo magnetinių sūkurių judėjimas superlaidininke, eksperimentuose pasireiškiantis įtampos laipteliais voltamperinėje darinio charakteristikoje, gali atsirasti esant FL >> Fp netgi tuomet, kai sūkurių kanalas nėra visiškai užpildytas Abrikosovo magnetiniais sūkuriais, dėl ko sūkuriai silpnai sąveikauja tarpusavyje.
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van JAARSVELD, J. P. J., A. P. C. HOLTEN, A. ELSENAAR, R. R. TRIELING, and G. J. F. van HEIJST. "An experimental study of the effect of external turbulence on the decay of a single vortex and a vortex pair." Journal of Fluid Mechanics 670 (February 22, 2011): 214–39. http://dx.doi.org/10.1017/s0022112010005197.
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This study is concerned with the effect of external turbulence on the decay of vortices. Single vortices and vortex pairs were generated with wing(s) mounted in the sidewalls of a wind tunnel. The distance between the two vortices could be adjusted such that they just touched each other or overlapped. The intensity of the turbulence could be controlled with a turbulence grid. The development of the vortex was measured at a number of downstream stations with particle image velocimetry for a range of wing settings. The results indicate that the single vortex can be described by the ‘two length scales’ model of Jacquin, Fabre & Geffroy (AIAA, vol. 1038, 2001, p. 1). A vortex core, which decays like a Lamb–Oseen vortex, is embedded in a region with an almost constant radius and a much lower azimuthal velocity that obeys a ~r−β power law, with r being the radius measured from the vortex centre. For the turbulence levels and the test section length used in this study, the single-vortex behaviour is independent of the external turbulence and in contrast with the decay of the vortex pair that strongly depends on the external turbulence. In the initial stages of the vortex pair evolution, the vortices decay due to cancellation of vorticity at the symmetry plane. At a later stage, Crow oscillations are observed, followed by a breakdown of the vortices. This vortex breakdown might be due to direct turbulent action. The observed behaviour is in agreement with the theoretical model of Crow & Bate (J. Aircraft, vol. 13, 1976, p. 476).
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Trewartha, Daniel, Waseem Kamleh, and Derek Leinweber. "Evidence that centre vortices underpin dynamical chiral symmetry breaking in SU(3) gauge theory." Physics Letters B 747 (July 2015): 373–77. http://dx.doi.org/10.1016/j.physletb.2015.06.025.
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MONTGOMERY, MICHAEL T., VLADIMIR A. VLADIMIROV, and PETR V. DENISSENKO. "An experimental study on hurricane mesovortices." Journal of Fluid Mechanics 471 (November 5, 2002): 1–32. http://dx.doi.org/10.1017/s0022112002001647.
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Mesovortices in the eyewall region of a hurricane are intriguing elements of the hurricane engine. In-situ measurements of them are sparse, however, and our understanding of their overall role in the physics of a hurricane is incomplete. To further understand their dynamics an experimental apparatus using a homogeneous fluid (water) has been constructed to emulate the lower tropospheric flow of the hurricane eye/eyewall region.For experimental configurations possessing a central aspect ratio less than unity, a primary and secondary circulation similar to the in flow layer of an intense hurricane, and a similar radius-to-width ratio of the curvilinear shear layer bordering the eye and eyewall region, the flow supports two primary quasi-steady vortices and secondary intermittent vortices. The vortices form through Kelvin–Helmholtz instability of the curvilinear shear layer bordering the slowly upwelling fluid in the centre and the converging fluid from the periphery. The primary vortices are maintained by convergence of circulation from the periphery and merger of secondary vortices spawned along the shear layer.The horizontal flow field is measured using a particle image velocimeter. Despite the relatively strong secondary circulation through the parent vortex the horizontal flow is found to be approximately uniform in the direction parallel to the rotation axis. The peak tangential velocity is found to occur in the mesovortices and is roughly 50% greater than the parent vortex that supports them. The measurements provide insight into recent observations of excessive wind damage in landfalling storms and support the hypothesis that intense storms contain coherent vortex structures in the eyewall region with higher horizontal wind speeds locally than the parent hurricane.
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FABRE, DAVID, and STÉPHANE LE DIZÈS. "Viscous and inviscid centre modes in the linear stability of vortices: the vicinity of the neutral curves." Journal of Fluid Mechanics 603 (April 30, 2008): 1–38. http://dx.doi.org/10.1017/s0022112008000578.
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In a previous paper, We have recently that if the Reynolds number is sufficiently large, all trailing vortices with non-zero rotation rate and non-constant axial velocity become linearly unstable with respect to a class of viscous centre modes. We provided an asymptotic description of these modes which applies away from the neutral curves in the (q, k)-plane, where q is the swirl number which compares the azimuthal and axial velocities, and k is the axial wavenumber. In this paper, we complete the asymptotic description of these modes for general vortex flows by considering the vicinity of the neutral curves. Five different regions of the neutral curves are successively considered. In each region, the stability equations are reduced to a generic form which is solved numerically. The study permits us to predict the location of all branches of the neutral curve (except for a portion of the upper neutral curve where it is shown that near-neutral modes are not centre modes). We also show that four other families of centre modes exist in the vicinity of the neutral curves. Two of them are viscous damped modes and were also previously described. The third family corresponds to stable modes of an inviscid nature which exist outside of the unstable region. The modes of the fourth family are also of an inviscid nature, but their structure is singular owing to the presence of a critical point. These modes are unstable, but much less amplified than unstable viscous centre modes. It is observed that in all the regions of the neutral curve, the five families of centre modes exchange their identity in a very intricate way. For the q vortex model, the asymptotic results are compared to numerical results, and a good agreement is demonstrated for all the regions of the neutral curve. Finally, the case of ‘pure vortices’ without axial flow is also considered in a similar way. In this case, centre modes exist only in the long-wave limit, and are always stable. A comparison with numerical results is performed for the Lamb–Oseen vortex.
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Druzhinin, O. A. "On the two-way interaction in two-dimensional particle-laden flows: the accumulation of particles and flow modification." Journal of Fluid Mechanics 297 (August 25, 1995): 49–76. http://dx.doi.org/10.1017/s0022112095003004.
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The evolution of two-dimensional regular flows laden with solid heavy particles is studied analytically and numerically. The particulate phase is assumed to be dilute enough to neglect the effects of particle-particle interactions. Flows with large Reynolds and Froude numbers are considered, when effects related to viscous dissipation and gravity are negligible. A Cauchy problem is solved for an initially uniform distribution of particles with Stokes (St) and Reynolds (Rep) numbers of order unity in several types of flows representing steady solutions of the two-dimensional Euler equations. We consider flows in the vicinity of the hyperbolic stagnation point (with a uniform strain and zero vorticity) and the elliptic stagnation point (where vorticity is uniform), a circular vortex (with vorticity depending on the radius) and Stuart vortex flow. Analytical solutions are obtained, for the case of sufficiently small St, describing the accumulation of particles and corresponding modification of the fluid flow. Solutions derived show that the concentration of particles, although remaining uniform, decreases at the elliptic stagnation point and grows at the hyperbolic point. Owing to the coupling between the particulate and fluid dynamics, the flow vorticity is reduced at the elliptic point, while flow strain rate is enhanced at the hyperbolic point. Solutions obtained for the circular vortex show that the accumulation of particles proceeds in the form of a travelling wave. The concentration grows locally, forming the crest of the wave which propagates away from the vortex centre. Owing to the influence of the particulate on the carrier flow, the vorticity is reduced in the vortex centre. At the location of the crest the gradient of the flow grows owing to the drag forces between the fluid and particles and a vorticity peak is generated. Analytical solutions are also obtained for a chain of particle-laden Stuart vortices. Owing to the coupling effects, the concentration is diminished and the vorticity is reduced at the centres of the vortices. A sheet of increased concentration and vorticity is formed extending from the braid region to the periphery of the vortices, and the flow strain in the braid region is enhanced. Results of numerical simulations performed for St = 0.5 show good agreement with analytical solutions.
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Li, Gen, Ulrike K. Müller, Johan L. van Leeuwen, and Hao Liu. "Fish larvae exploit edge vortices along their dorsal and ventral fin folds to propel themselves." Journal of The Royal Society Interface 13, no. 116 (March 2016): 20160068. http://dx.doi.org/10.1098/rsif.2016.0068.
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Larvae of bony fish swim in the intermediate Reynolds number ( Re ) regime, using body- and caudal-fin undulation to propel themselves. They share a median fin fold that transforms into separate median fins as they grow into juveniles. The fin fold was suggested to be an adaption for locomotion in the intermediate Reynolds regime, but its fluid-dynamic role is still enigmatic. Using three-dimensional fluid-dynamic computations, we quantified the swimming trajectory from body-shape changes during cyclic swimming of larval fish. We predicted unsteady vortices around the upper and lower edges of the fin fold, and identified similar vortices around real larvae with particle image velocimetry. We show that thrust contributions on the body peak adjacent to the upper and lower edges of the fin fold where large left–right pressure differences occur in concert with the periodical generation and shedding of edge vortices. The fin fold enhances effective flow separation and drag-based thrust. Along the body, net thrust is generated in multiple zones posterior to the centre of mass. Counterfactual simulations exploring the effect of having a fin fold across a range of Reynolds numbers show that the fin fold helps larvae achieve high swimming speeds, yet requires high power. We conclude that propulsion in larval fish partly relies on unsteady high-intensity vortices along the upper and lower edges of the fin fold, providing a functional explanation for the omnipresence of the fin fold in bony-fish larvae.
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Lühr, H., M. Lockwood, P. E. Sandholt, T. L. Hansen, and T. Moretto. "Multi-instrument ground-based observations of a travelling convection vortices event." Annales Geophysicae 14, no. 2 (February 29, 1996): 162–81. http://dx.doi.org/10.1007/s00585-996-0162-z.
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Abstract. A coordinated ground-based observational campaign using the IMAGE magnetometer network, EISCAT radars and optical instruments on Svalbard has made possible detailed studies of a travelling convection vortices (TCV) event on 6 January 1992. Combining the data from these facilities allows us to draw a very detailed picture of the features and dynamics of this TCV event. On the way from the noon to the drawn meridian, the vortices went through a remarkable development. The propagation velocity in the ionosphere increased from 2.5 to 7.4 km s–1, and the orientation of the major axes of the vortices rotated from being almost parallel to the magnetic meridian near noon to essentially perpendicular at dawn. By combining electric fields obtained by EISCAT and ionospheric currents deduced from magnetic field recordings, conductivities associated with the vortices could be estimated. Contrary to expectations we found higher conductivities below the downward field aligned current (FAC) filament than below the upward directed. Unexpected results also emerged from the optical observations. For most of the time there were no discrete aurora at 557.7 nm associated with the TCVs. Only once did a discrete form appear at the foot of the upward FAC. This aurora subsequently expanded eastward and westward leaving its centre at the same longitude while the TCV continued to travel westward. Also we try to identify the source regions of TCVs in the magnetosphere and discuss possible generation mechanisms.
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Tchieu, Andrew A., Eva Kanso, and Paul K. Newton. "The finite-dipole dynamical system." Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 468, no. 2146 (May 9, 2012): 3006–26. http://dx.doi.org/10.1098/rspa.2012.0119.
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The notion of a finite dipole is introduced as a pair of equal and opposite strength point vortices (i.e. a vortex dipole) separated by a finite distance. Equations of motion for N finite dipoles interacting in an unbounded inviscid fluid are derived from the modified interaction of 2 N independent vortices subject to the constraint that the inter-vortex spacing of each constrained dipole, ℓ, remains constant. In the absence of all other dipoles and background flow, a single dipole moves in a straight line along the perpendicular bisector of the line segment joining the two point vortices comprising the dipole, with a self-induced velocity inversely proportional to ℓ. When more than one dipole is present, the velocity of the dipole centre is the sum of the self-induced velocity and the average of the induced velocities on each vortex comprising the pair due to all the other dipoles. Each dipole orients in the direction of shear gradient based on the difference in velocities on each of the two vortices in the pair. Several numerical experiments are shown to illustrate the interactions between two and three dipoles in abreast and tandem configurations. We also show that equilibria (multi-poles) can form as a result of the interactions, and we study the stability of polygonal equilibria, showing that the N =3 case is linearly stable, whereas the N >3 case is linearly unstable.
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BURTON, G. R., and J. NYCANDER. "Stationary vortices in three-dimensional quasi-geostrophic shear flow." Journal of Fluid Mechanics 389 (June 25, 1999): 255–74. http://dx.doi.org/10.1017/s0022112099005029.
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An existence theorem for localized stationary vortex solutions in an external shear flow is proved. The flow is three-dimensional and quasi-geostrophic in an unbounded domain. The external flow is unidirectional, with linear horizontal and vertical shear. The flow conserves an infinite family of Casimir integrals. Flows that have the same value of all Casimir integrals are called isovortical flows, and the potential vorticity (PV) fields of isovortical flows are stratified rearrangements of one another. The theorem guarantees the existence of a maximum-energy flow in any family of isovortical flows that satisfies the following conditions: the PV-anomaly must have compact support, it must have the same sign everywhere, and this sign must be the same as the sign of the external horizontal shear over the vertical interval to which the support of the PV-anomaly is confined. This flow represents a stationary and localized vortex, and the maximum-energy property implies that the vortex is stable. The PV-anomaly decreases monotonically outward from the vortex centre in each horizontal plane, but apart from this the profile is arbitrary.
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Duda, Daniel, Vitalii Yanovych, Václav Uruba, Martin Němec, and Pavel Žitek. "Vortices inside a single-stage axial air turbine captured by Particle Image Velocimetry." MATEC Web of Conferences 328 (2020): 05002. http://dx.doi.org/10.1051/matecconf/202032805002.
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Particle Image Velocimetry (PIV) is an experimental method of fluid research resulting into a spatially resolved two-dimensional velocity field. We measured the velocity inside a single-stage axial test turbine at the Czech Aerospace Research Centre. We studied the axial × tangential plane just behind the rotor wheel at the tip radius under three regimes: underloaded, designed and overloaded ones. We found individual vortices in the instantaneous snapshots and we analyze their properties statistically.
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., Sutrisno, Tri Agung Rochmat, Setyawan Bekti Wibowo, Sigit Iswahyudi, Caesar Wiratama, and Widia Kartika. "The Flow Visualization CFD Studies of the Fuselage and Rolled-up Vortex Effects of the Chengdu J-10-like Fighter Canard." Modern Applied Science 12, no. 2 (January 30, 2018): 148. http://dx.doi.org/10.5539/mas.v12n2p148.
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Fighter aircrafts with high maneuverability and swiftness are due to fuselage effects, caused by canard-fuselage-main wing configuration. Even though the flows around fighters are highly complex, mostly they create rolled-up vortices capable to delay stalls and increase maximum lifts (Calderon, Wang & Gursul, 2012; Mitchell & Delery, 2001; Boelens, 2012; Chen, Liu, Guo & Qu, 2015). The vortex dynamics analysis method employment is introduced, in this case we focus only on the fighter canard. It characterizes the vortex core, develops the pitching moment & main wing total lift, and exploits the vortex centre visualization, the strength, negative surface pressure and its trajectory.This paper explains the influence of the fighter fuselage, it generates rolled-up vortex effects, causes the flow deflected by the fighter fuselage head, strengthen the vortex centre to become vortex core. Above the aircraft head, due to the curved contour head effect, the second vortex centers are developed makes the vortex center above the head more dynamic. Comparing with fighter without fuselage, the flow property changes, for Chengdu J-10-like model with fuselage, are concentrated at the canard leading edge, where the negative pressures are stronger, since the maximum axial velocities of the vortex centre are higher, and give more distinctive vortex breakdown locations.
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Yu, S. C. M., Y. X. Hou, and S. C. Low. "The flow characteristics of a confined square jet with mixing tabs." Proceedings of the Institution of Mechanical Engineers, Part G: Journal of Aerospace Engineering 212, no. 2 (February 1, 1998): 63–76. http://dx.doi.org/10.1243/0954410981532144.
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The flow characteristics of a confined square jet with mixing tabs have been determined by measurements obtained using a two-component laser Doppler anemometer at a Reynolds number of 1.026 × 105 (based on the exit hydraulic diameter, DH = 60 mm, and bulk mean velocity, Ur, of the stream at 1.71 m/s). Both tabs of rectangular and triangular shapes are considered with the same height-breadth ratio ( h/b = 1.35) and with their apex leaning downstream. Altogether four tabs have been used, with one tab each located at the centre of each side wall at the exit plane. Each tab is found to produce a dominant pair of counter-rotating streamwise vortices. The combined effects of the four tabs bifurcate the jet into four ‘fingers’, resulting in a significant increase in entrainment at the downstream locations. The strength of the streamwise vorticity generated by the rectangular tabs is some 30 per cent higher than the triangular ones and decays faster with downstream distance. This appears due to a larger tab surface area which creates a larger pressure differential across the rectangular tab than the triangular tab. The region of high turbulent kinetic energy is found firstly at the locations where the streamwise vortices stretch the normal vortices and subsequently at locations where the streamwise vortices break down, resulting in significant mixing enhancement. Finally, the effects of the so-called secondary tabs have also been examined and are found to enhance the mixing further. The orientation of the secondary tabs is, however, crucial for the mixing enhancement to occur.
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ISHIHARA, NORIO, and SHIGEO KIDA. "Dynamo mechanism in a rotating spherical shell: competition between magnetic field and convection vortices." Journal of Fluid Mechanics 465 (August 25, 2002): 1–32. http://dx.doi.org/10.1017/s0022112002008935.
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A strong axial magnetic dipole field with magnetic energy 15 times larger than the kinetic energy of thermal convection is realized by a direct numerical simulation of the magnetohydrodynamic equation of an electrically conducting Boussinesq fluid in a rotating spherical shell which is driven by a temperature difference between the outer and inner boundaries against a gravity force pointed towards the system centre. Cyclonic and anticyclonic convection vortices are generated and play a primary role in the magnetic field intensification. The magnetic field is enhanced through the stretching of magnetic lines in four particular parts of the convection fields, namely inside anticyclones, between cyclones and their western neighbouring anticyclones at middle as well as low latitudes, and between anticyclones and the outer boundary. A ‘twist-turn’ loop of intense magnetic flux density is identified as a fundamental structure which yields dominant contributions both to the toroidal and poloidal components of the longitudinally averaged magnetic field. Various types of competitive interaction between the magnetic field and convection vortices are observed. Among these, a creation-and-annihilation cycle in a statistically equilibrium state is particularly important. It is composed of three sequentially recurrent dynamical processes: the generation of convection vortices by the Rayleigh–Bénard instability, the growth of anticyclones and the intensification of magnetic field by a concentrate-and-stretch mechanism, and the breakdown of vortices by the Lorentz force followed by diminution of the magnetic field. The energy transfer from the velocity to the magnetic fields takes place predominantly in this dynamical cycle.
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HETTEL, M., F. WETZEL, P. HABISREUTHER, and H. BOCKHORN. "Numerical verification of the similarity laws for the formation of laminar vortex rings." Journal of Fluid Mechanics 590 (October 15, 2007): 35–60. http://dx.doi.org/10.1017/s0022112007007677.
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From analytical investigations it is well known that the roll-up of an inviscid plane vortex sheet which separates at the edge of a body is a self-similar process which can be described by scaling laws. Unlike plane vortices, ring vortices have a curved rotational axis. For this special vortex type experimental investigations as well as calculations in the literature suggest that the scaling laws are only partially valid. The main goal of this work is to clarify how far these similarity or scaling laws are also valid for the formation of viscid laminar vortex rings. Therefore, the formation process of laminar vortex rings was investigated numerically using a CFD (computational-fluid-dynamics) code. The calculations refer to an experimental setup for which detailed experimental data are available in the literature. In this setup, laminar ring vortices are generated by ejecting water from a circular tube into a quiescent environment by means of a piston. First, a case based on a constant piston velocity was investigated. Comparing calculated and measured data yields a very good agreement. Further calculations were made when forcing the velocity of the piston by three different time-dependent functions. The results of these calculations show that the formation laws for inviscid plane vortices are also valid for the formation process of viscid ring vortices. This applies to the normalized axial and radial position of the vortex centre as well as the normalized diameter of the vortex spiral. However, the similarity laws are valid only if the process is considered in a special frame of reference which moves in conjunction with the front of the jet and if the starting time of the formation process with respect to the starting time of the ejection is taken into account. Additionally, the formation of a ring vortex, which occurs during the start-up process of a free jet flow, was calculated. The results confirm a dependence for the motion of the jet front, which is known from analytical considerations and allows some interesting features to be identified.
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LYUBIMOVA, T. P., D. V. LYUBIMOV, V. A. MOROZOV, R. V. SCURIDIN, H. BEN HADID, and D. HENRY. "Stability of convection in a horizontal channel subjected to a longitudinal temperature gradient. Part 1. Effect of aspect ratio and Prandtl number." Journal of Fluid Mechanics 635 (September 10, 2009): 275–95. http://dx.doi.org/10.1017/s0022112009007587.
Full textAbstract:
The paper deals with the numerical investigation of the steady convective flow in a horizontal channel of rectangular cross-section subjected to a uniform longitudinal temperature gradient imposed at the walls. It is shown that at zero Prandtl number the solution of the problem corresponds to a plane-parallel flow along the channel axis. In this case, the fluid moves in the direction of the imposed temperature gradient in the upper part of the channel and in the opposite direction in the lower part. At non-zero values of the Prandtl number, such solution does not exist. At any small values of Pr all three components of the flow velocity differ from zero and in the channel cross-section four vortices develop. The direction of these vortices is such that the fluid moves from the centre to the periphery in the vertical direction and returns to the centre in the horizontal direction. The stability of these convective flows (uniform along the channel axis) with regard to small three-dimensional perturbations periodical in the direction of the channel axis is studied. It is shown that at low values of the Prandtl number the basic state loses its stability due to the steady hydrodynamic mode related to the development of vortices at the boundary of the counter flows. The growth of the Prandtl number results in the strong stabilization of this instability mode and, beyond a certain value of the Prandtl number depending on the cross-section aspect ratio, a new steady hydrodynamic instability mode becomes the most dangerous. This mode is characterized by the localization of the perturbations near the sidewalls of the channel. At still higher values of the Prandtl number, the spiral perturbations (rolls with axis parallel to the temperature gradient) become the most dangerous modes, at first the oscillatory spiral perturbations and then the Rayleigh-type steady spiral perturbations. The influence of the channel width on these different instabilities is also emphasized.
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SAHNI, ONKAR, JOSHUA WOOD, KENNETH E. JANSEN, and MICHAEL AMITAY. "Three-dimensional interactions between a finite-span synthetic jet and a crossflow." Journal of Fluid Mechanics 671 (February 9, 2011): 254–87. http://dx.doi.org/10.1017/s0022112010005604.
Full textAbstract:
A complementary experimental and numerical investigation was performed to study the three-dimensional flow structures and interactions of a finite-span synthetic jet in a crossflow at a chord-based Reynolds number of 100,000 and a 0° angle of attack. Six blowing ratios in the range of 0.2–1.2 were considered. Experiments were conducted on a finite wing with a cross-sectional profile of NACA 4421, where particle-image velocimetry data were collected at the centre jet. To complement the experiments, three-dimensional numerical simulations were performed, where the numerical set-up matched not only the physical parameters (e.g. free stream) but also the physical dimensions (e.g. orientation and location of the jet. For the low blowing ratio cases, spatial non-uniformities developed, due to the finite span of the slit, which led to the formation of small and organized secondary structures or a streak-like pattern in the mean flow. On the other hand, for the high blowing ratio range, turbulent vortical structures were dominant, leading to larger spanwise structures, with a larger spanwise wavelength. Moreover, the phase-locked flow fields exhibited a train of counter-rotating coherent vortices that lifted off the surface as they advected downstream. In the mid-blowing ratio range, combined features of the low range (near the slit) and high range (in downstream locations) were found, where a pair of counter-rotating vortices issued in the same jet cycle collided with each other. In all cases, the spanwise extent of the secondary coherent structures reduced with downstream distance with a larger decrease at higher blowing ratios. Similar observations were made in earlier studies on finite-span synthetic jets in quiescent conditions.
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SUMNER, D., S. J. PRICE, and M. P. PAÏDOUSSIS. "Flow-pattern identification for two staggered circular cylinders in cross-flow." Journal of Fluid Mechanics 411 (May 25, 2000): 263–303. http://dx.doi.org/10.1017/s0022112099008137.
Full textAbstract:
The flow around two circular cylinders of equal diameter, arranged in a staggered configuration, was investigated using flow visualization and particle image velocimetry for centre-to-centre pitch ratio P/D = 1[ratio ]0 to 5.0 and angle of incidence. α = 0° to 90°. Experiments were conducted within the low subcritical Reynolds number regime, from Re = 850 to 1900. Nine flow patterns were identified, and processes of shear layer reattachment, induced separation, vortex pairing and synchronization, and vortex impingement, were observed. New insight was gained into previously published Strouhal number data, by considering the flow patterns involved. The study revealed that vortex shedding frequencies are more properly associated with individual shear layers than with individual cylinders; more specifically, the two shear layers from the downstream cylinder often shed vortices at different frequencies.
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Mees, Philip A. J., K. Nandakumar, and J. H. Masliyah. "Secondary instability of flow in a curved duct of square cross-section." Journal of Fluid Mechanics 323 (September 25, 1996): 387–409. http://dx.doi.org/10.1017/s0022112096000973.
Full textAbstract:
Experiments and simulations of a travelling wave state of incompressible Newtonian flow in a curved duct of square cross-section are presented. The travelling wave mode develops from the well-documented steady four-cell flow state and is characterized by oscillations of the two Dean vortices near the centre of the outer wall.The oscillations were induced by a carefully positioned pin at 5° from the inlet of the curved section along the symmetry line of the cross-section. It was shown that the travelling wave state is characteristic for curved duct flow and that the pin made it possible to observe the oscillations within the 270° long curved duct. Travelling waves were observed at flow rates above Dn = 170 (Dn = Re/(R/a)1/2, where Re is the Reynolds number, R is the radius of curvature of the duct and a is the duct dimension. The curvature ratio, R/a, is 15.1).If no other disturbances are imposed, the oscillations are the result of the selective amplification of random disturbances in the flow, leading to a broad frequency spectrum. The travelling wave was found to lock in to an imposed periodic disturbance at a selected frequency. The flow structure of the locked state was investigated in detail, using flow visualization and a one-component laser Doppler anemometer to measure streamwise or spanwise velocities. Direct numerical simulations using the package CFDS-FLOW3D are in very good agreement with the experiments and confirm the existence of a fully developed, streamwise-periodic travelling wave state.The inflow region between the two Dean vortices, which transports low-speed fluid away from the outer wall, creates strongly inflectional spanwise profiles of the streamwise velocity. Similarities with twisting vortices in a curved channel and sinuous oscillations of Görtler vortices show that the travelling waves observed here result from a secondary shear instability of these spanwise inflectional profiles.
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Jevnikar, Aleks. "An existence result for the mean-field equation on compact surfaces in a doubly supercritical regime." Proceedings of the Royal Society of Edinburgh: Section A Mathematics 143, no. 5 (September 25, 2013): 1021–45. http://dx.doi.org/10.1017/s030821051200042x.
Full textAbstract:
We consider a class of variational equations with exponential nonlinearities on a compact Riemannian surface, describing the mean-field equation of the equilibrium turbulence with arbitrarily signed vortices. For the first time, we consider the problem with both supercritical parameters and we give an existence result by using variational methods. In doing so, we present a new Moser–Trudinger-type inequality under suitable conditions on the centre of mass and the scale of concentration of both eu and e−u, where u is the unknown function in the equation.
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Alexander, Lamoire J. D., and George A. Aldworth. "Model test of a pumping station wet well." Canadian Journal of Civil Engineering 14, no. 1 (February 1, 1987): 24–32. http://dx.doi.org/10.1139/l87-004.
Full textAbstract:
A case study is presented describing the sump model test of a wet well in an existing sewage pumping station at the Regional Municipality of Ottawa–Carleton Green Creek Pollution Control Centre in Ottawa, Ontario. The purpose of the test was to determine whether the existing sump could accommodate projected flow increases of up to 83% associated with the proposed expansion of the plant. It was anticipated that the (low increases could cause the formation of localized flow disturbances such as vortices and eddies, which might induce unacceptable cavitation and (or) vibrations in the proposed new larger pumps. The sump model was constructed entirely of Plexiglas to facilitate flow visualization enhanced by a dye tracer. Of particular interest was the use of a laser Doppler anemometer to measure velocity distributions at critical locations such as the pump intakes. This provided a significant improvement in accuracy and convenience over more conventional velocity-recording devices such as a pitot tube or hot-wire anemometer. The methodology and considerations in the selection of the model scale are described. Key words: model, sump, similitude, vortices, pumps.